For efficient transport and storage, and to improve the keeping properties of the product, agricultural products like hay, straw and silage may be compressed into bales of a parallelepiped shape, which are known as ‘square’ bales. After the compression of the bale material the shape and the compression of the bale is maintained by binding the bale with twines that are looped around the compressed bale material. The ends of the twine are then knotted together.
Typically, compression of the bale material is performed by a reciprocating plunger press baler. A typical baler of this type is described in U.S. Pat. Nos. 4,074,623 and 4,142,746 to Hesston Corporation. This baler machine includes a baling chamber comprising an open-ended channel through which bale material is forced by a reciprocating plunger. The plunger is driven in a substantially linear direction between two end positions comprising respectively a withdrawn position in front of the baling chamber and an extended position in which it extends into the baling chamber. When the plunger is in the withdrawn position the baling chamber is loaded with the bale material to be compressed. The plunger is then driven into the baling chamber so that this new material is compressed against a body of compressed material already in the baling chamber. Any newly compacted material that is added to the already compacted material in the chamber is called a ‘wad’. The friction of the compressed material with the walls of the baling chamber provides a resistive force allowing for compression of the new material that is introduced into the baling chamber in front of the plunger.
After compression, the newly compressed material and the compressed material already in the baling chamber are moved together towards the outlet end of the channel until the plunger reaches its fully extended end position. The plunger then moves in the opposite direction toward its withdrawn position so that the baling chamber can be reloaded with new material to be compressed.
The friction within the baling chamber between the already formed bale and the walls of the channel can be regulated for instance by pressing the side walls and/or the top panels of the baling chamber against the compressed material with different levels of force.
The bale is held in compression after leaving the machine by binding the bale with twines that are tied around the body of compressed material. In some machines, a single spool process is used in which each binding is formed with twine that is taken from a single spool and looped around the entire circumference of the bale, then tied with a single knot. Such a process is described in U.S. Pat. No. 3,895,571, which includes a mechanism for releasing the tension in the twine during knotting so as to avoid problems while knotting the twine.
Another example of a machine that uses a single spool process is described in EP0392627A.
This machine includes a mechanism for reducing the bale pressure at the start of the bale forming process to allow the binding twine to slide more easily between the bale material and the previously formed bale that is still in the baling chamber.
Another machine that uses a single spool process is described in DE4031695A. This machine includes a mechanism for reducing the bale pressure at the start of the twine knotting process to allow the binding twine to be knotted more easily.
Other baling machines use a twin spool process in which each binding is formed using twines from two spools, which are tied with two knots at opposite ends of the bale. One advantage of the twin spool process is that the bale can be compressed to a higher compaction pressure because the twine does not have to be passed between the newly compacted bale and the previously formed bale. However, this increased compacting pressure also increases the risk of the twines breaking when the bale is ejected from the baling chamber. The twin spool binding process will now be described in more detail.
At the start of the baling process two lengths of twine from spools on opposite sides of the baling chamber are connected to one another by tying the ends of the twines together.
As the bale material is compacted in the baling chamber the spools feed twine to the baling chamber on either side of the bale material. On one side of the baling chamber the twine passes through the tip of a baling needle. When the body of bale material has reached its full length, between two successive compressing strokes, the needle brings the twine as a loop to the other side of the baling chamber. A knotter device then knots the twine, joining an end of the twine loop that was brought around the compressed bale by the needle to an end of the twine that was supplied by the spool on other side of the baling chamber (on the same side as the knotter). A second knot is also formed for the start of the next bale. The needle is then retracted and a new bale is started.
The pressure applied to the material in the baling chamber during the compression stroke is typically 3 to 4 bar for a bale with a typical compression surface of 90*120 cm. The knotted twine used in this type of baling machine typically has a breaking strength of 200 kgf. Six knotted twines can therefore hold about 6*2*200=2400 kgf. In a conventional bale with a compression surface of 90*120 cm this results in a holding pressure of only 0.22 bar. This places an upper limit on the compression pressure that can be applied to the bale. If a higher compression level is to be maintained, more binding twines have to be used.
There is generally some expansion of the bale as it is ejected from the channel of the baling chamber and this expansion has to be taken into account when designing the baling machine to ensure that the twines are able to maintain the compression of the bale without breaking. However, the amount of expansion is not uniform. Some materials such as dry straw and grass are more elastic than others and tend to expand more. There is therefore a greater risk that the twines will break when baling such materials.
To reduce the risk of breakage when baling highly elastic bale materials, in praxis the overall compression level is sometimes reduced. However, this reduces the density and mass of the bale, which is generally undesirable.
Alternatively, the twine loops tied around the compressed material can be made by design slightly longer than the circumference of the compressed bale while it is in the baling chamber. Then, when the bale leaves the baling chamber it expands to a size determined by the slightly greater length of the twine. Due to this expansion, the pressure in the bale falls to a value that can be withstood by the twines. However, this also has a negative impact on the mass and density of the bale and it results in a lower level of compression for all materials, including less elastic materials for which a lower level of compression is not needed.
Recently, the compression level that can be produced by baling machines that use the twin spool process has increased to typically about 6-10 bar, but the holding strength of the twine has hardly improved. The increased compression level places greater stress on the baling twines, particularly when binding materials that are relatively elastic, for example dry grass. We have found that with some materials and/or in certain weather conditions the pressure after expansion of the bale can be higher than the twines can withstand. As a result, we have found that the twines can break and the bales can then burst during or after leaving the baling chamber.
Attempts to reduce the problem described above have been described in U.S. Pat. No. 4,577,553 which includes a mechanism for increasing the length of the twine loops so that they are less likely to burst, particularly when using sisal twine. However this invention does not describe any possibility to adjust the increase of loop length in relation to the expansion of the material. Furthermore, it is complicated and not reliable since the hook for pulling extra twine length has to grab the twine from the bale, while the position of the twine on the bale can vary widely.
Another attempt to address the problem is described in WO 2013017229. In praxis it shows that this method under certain field and material conditions is difficult to adjust and gives poorly shaped bale ends and sometimes overly dense bales because the last part of the bale is compressed at a lower level.